In the art of hydrostatics, oil or fluid is pumped by mechanical hydrostatic pumps for the purpose of causing a hydrostatic motor to revolve, a hydrostatic cylinder to extend, or for other useful purposes. A common aspect of many tractors, earthmoving machines and the like is a hydrostatic transmission. In its most basic form, a hydrostatic transmission consists of a hydrostatic pump which is normally driven by an internal combustion engine, and provides a source of pressurized oil flow which causes one of more hydrostatic motors to rotate. The rotation of these one or more hydrostatic motors will cause the machine to travel forward or reverse as commanded by the operator of the machine.
In contrast, to hydraulic transmissions, hydrostatic transmissions operate in what is known as a closed circuit. In a closed circuit, pressurized oil from a pump is piped directly (or through valving) into a motor. Oil is returned from the motor to the pump. This system is known as closed circuit because hydrostatic fluid (i.e. hydrostatic oil) normally circulates in a closed path between the pump and the motor without passing into an oil reservoir on each pass. This closed circuit differs from an open circuit where a pump draws hydraulic fluid from a fluid reservoir, the fluid is piped to the motor, and then the return fluid from the motor is piped back into the fluid reservoir. Even in a closed circuit, a small reservoir and a charge pump will be used to collect the small amount of fluid which leaks out of the circuit and to replace that flow so that the closed circuit remains full of fluid at all times.
When a hydrostatic transmission is operated under heavy loads for an extended period of time, it is possible for the oil, which is pumped in the closed circuit, to become heated to an extent which may not be desirable. This heating occurs due to friction and other processes. Hydrostatic fluid may degrade more quickly when maintained at excessive temperatures, thus requiring premature replacement of the hydrostatic fluid. Further, elevated temperatures, hydrostatic fluid may lose certain lubricating properties including, but not limited to, viscosity. When a hydrostatic fluid loses viscosity, it compromises the fluid's ability to prevent damaging wear to the hydrostatic machinery. In order to remove heated oil from the closed circuit, a “controlled leak” or flushing system is employed to remove fluid from the circuit. This oil is then cooled and replaced back through a charge pump.
Usually the flushing system uses a flush valve that is connected to both the high pressure and low pressure fluid paths on a closed hydrostatic circuit. The flush valve is configured to select the lower pressure line of the two hydrostatic circuit pressure lines in the closed circuit. The flush valve is further connected to an orifice, or to a pressure relief valve, or to some combination of both. This relief valve serves to relieve circuit charge pressure and to control the release of hydrostatic fluid from the circuit. The orifice serves to provide a minimal flushing flow of hydrostatic fluid through the circuit to maintain the fluid at an appropriate temperature.
Hydrostatic systems include several deficiencies. For example, current circuit flushing systems that incorporate a flush valve and a relief valve cannot be intelligently controlled. Because the system is not intelligently controlled, the circuit flushing action occurs whenever the transmission is operational. Therefore, a circuit flushing system must be sized to flush an adequate flow of hydrostatic fluid under worst case operating conditions. Consequently, the volume of circuit flushing flow will therefore always be as high as the flow required under the most severe operating conditions. Because the circuit flushing flow is typically higher than desired, a larger charge pump is required which will consume more energy and result in increased system energy losses.
Circuit flushing flow also causes energy and system efficiency losses known as parasitic losses at low speeds and/or low temperature conditions. Because the fluid in the circuit is pressurized, circuit flushing flow causes a frictional loss or waste of hydrostatic energy. This loss ultimately requires more power from the internal combustion engine and higher fuel consumption than would be otherwise required. Conversely, the circuit flushing flow must be maximized as the engine speed is increased or the hydrostatic fluid will become too hot and in danger of dropping below a minimum desired viscosity, e.g., 10 sCt.